Cisco Gigabit Ethernet Switch Module for HP p-Class BladeSystem Software Configuration Guide

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Cisco Gigabit Ethernet Switch Module for HP p-Class BladeSystem Software Configuration Guide
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Chapter 17 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
Leaving a Multicast Group
The router sends periodic multicast general queries, and the switch forwards these queries through all
ports in the VLAN. Interested blade servers respond to the queries. If at least one blade server in the
VLAN wishes to receive multicast traffic, the router continues forwarding the multicast traffic to the
VLAN. The switch forwards multicast group traffic only to those blade servers listed in the forwarding
table for that IP multicast group maintained by IGMP snooping.
When blade servers want to leave a multicast group, they can silently leave, or they can send a leave
message. When the switch receives a leave message from a blade server, it sends a group-specific query
to learn if any other devices connected to that interface are interested in traffic for the specific multicast
group. The switch then updates the forwarding table for that MAC group so that only those blade servers
interested in receiving multicast traffic for the group are listed in the forwarding table. If the router
receives no reports from a VLAN, it removes the group for the VLAN from its IGMP cache.
Immediate Leave
Immediate Leave is only supported on IGMP Version 2 hosts.
The switch uses IGMP snooping Immediate Leave to remove from the forwarding table an interface that
sends a leave message without the switch sending group-specific queries to the interface. The VLAN
interface is pruned from the multicast tree for the multicast group specified in the original leave message.
Immediate Leave ensures optimal bandwidth management for all blade servers on a switched network,
even when multiple multicast groups are simultaneously in use.
Note You should only use the Immediate Leave feature on VLANs where a single blade server is connected
to each port. If Immediate Leave is enabled in VLANs where more than one blade server is connected
to a port, some blade servers might inadvertently be dropped.
For configuration steps, see the “Enabling IGMP Immediate Leave” section on page 17-10.
IGMP Configurable-Leave Timer
In Cisco IOS Release 12.2(25)SEA and earlier, the IGMP snooping leave time was fixed at 5 seconds.
If membership reports were not received by the switch before the query response time of the query
expired, a port was removed from the multicast group membership. However, some applications require
a leave latency of less than 5 seconds.
In Cisco IOS Release 12.2(25)SED and later, you can configure the time that the switch waits after
sending a group-specific query to determine if hosts are still interested in a specific multicast group. The
IGMP leave response time can be configured from 100 to 5000 milliseconds. The timer can be set either
globally or on a per-VLAN basis. The VLAN configuration of the leave time overrides the global
configuration.
For configuration steps, see the “Configuring the IGMP Leave Timer” section on page 17-11.